Upconversion is generally the process of converting two or more low energy photons into one higher energy photon. Modern upconversion devices are broadly split into three classes. Traditionally, the lanthanides, also known as the rare earth elements, have been used in photon upconversion due to their unusual electronic states. However, the weak absorption of the low energy state requires high intensity excitation. This, in addition to the narrow line width of the absorption of the lanthanides along with the inability to tune the absorption or emission position (fixed by the material), makes this class of devices unfeasible for incoherent, low-intensity and broad-spectrum excitation such as sunlight.
The second method of upconversion is triplet-triplet annihilation which achieves upconversion for weak incoherent light. The state-of-the-art devices typically use heavy-metal complexes as the low energy photon absorbing material. It is, however, difficult to find stable sensitizers among this class of materials with strong absorption in the near infrared spectrum. The low energy state lifetime is reduced as the energy decreases, making these material a poor choice for upconversion of NIR photons. These materials also suffer from reduced energetic distance between absorption and emission due to the energy gap between triplet and singlet states in the sensitizer. The most efficient demonstrations have been in the solution form, which limits applications.
The third class of upconversion uses non-linear crystals to achieve upconversion. These systems are only efficient when exposed to very high-intensity, coherent light. In addition, there are considerable requirements on the optical geometry and form factor of the device.